1. Field of the Invention
The present invention relates to a webbing winding device having a pre-tensioner which tenses a passenger restraining webbing of a seat belt apparatus in a passenger restraining direction during an emergency deceleration of a vehicle cause by a vehicle collision or the like.
2. Description of the Related Art
Some types of webbing winding device for use in a vehicle seat belt unit have a pre-tensioner for removing play in the webbing by retracting the webbing so that it becomes tense during an emergency deceleration of a vehicle caused by a collision or the like. This type of webbing winding device has been described in, for example, Japanese Patent No. 2500192. The belt retractor which is a webbing winding device described in Japanese Patent No. 2500192 comprises a belt drum around which a belt web (webbing) is wound, a pinion connected to this belt drum via a free wheel connecting apparatus and gear transmitting means for transmitting rotation in one direction only, a rack which meshes with this pinion and a piston driving apparatus in which a piston is connected to this rack. When a vehicle is in a collision, the piston driving apparatus is actuated by gas generated by a gas generator so as to move the rack. The pinion is then rotated in a direction for winding up the belt web and the gear transmitting means increases the angular velocity of the rotation of the pinion and transmits it to the belt drum.
Further, even if the pre-tensioner is not actuated, the above described webbing winding device is continually urging the webbing winding shaft so as to reduce play in the webbing. However, the winding force for the webbing from the winding shaft cannot be made very strong in order to avoid imparting a feeling of strong pressure to a passenger from the webbing. AS a result, sometimes there is a slight play in the webbing and the amount of winding until the webbing becomes tense during an emergency such as a vehicle collision varies depending on the magnitude of this play. Therefore, in a conventional webbing winding device, in order to ensure that the webbing remains tense during an emergency such as a vehicle collision, the play in the webbing is assumed to be substantially at the maximum so that the webbing winding amount, which is the amount the webbing can be wound up by the pre-tensioner, is set as a large amount.
To increase the winding amount of the webbing by the pre-tensioner in the above described webbing winding device, it is necessary to increase the travelling stroke of the rack by the piston driving apparatus or increase the acceleration ratio from the gear transmission means so as to increase the distance rotated by the belt drum with respect to the distance traveled by the rack. However, if the movement stroke of the rack by the piston apparatus is increased, the lengths of the piston driving apparatus and rack are extended so that the size of the apparatus increases. Further, because the stages of the gears constituting the gear transmitting means need to be multiplied in order to increase the acceleration ratio of the gear transmission means, the size of the apparatus is enlarged.